Algae’s moving over the aquascape

[Stahre]

The title might be confusing, but in short, my tank is only 2 months old, so I understand that there will be algae. But what I found interesting is how it moves from rock to rock over time.

3 weeks ago, I had one big rock covered in brownish algae, it then turned to patches of green, then all was gone.

Now a completely different rock is experiencing the same.

Anyone know what causes this? I would have thought that maybe all rocks would be covered during the same period.

(The rock that’s has algae is dry rock, I have some live, that has not experienced any algae issues)

 

[blaxsun]

It's bacteria in the water column.